Rigid bodies seem to only have 3 DOF when particles overlap

Hello everyone,

I’m currently running LAMMPS 16Mar18 and using “fix rigid/small” with “fix pour molecule”. I am simulating granular flow in a quasi-two-dimensional hopper and when I use the following molecule template for the aforementioned fixes everything works fine:

Molecule file for dimers

2 atoms


1 -0.6 0.0 0.0
2 0.6 0.0 0.0


1 2
2 2


1 1.2
2 1.2


1 1.728
2 1.728

However, when I introduce a third particle with a coordinate of “0 0 0” (i.e. stuck between the two particles mentioned above), the rigid objects no longer rotate; the only translate as if the rotational degrees of freedom were
subtracted. I tried various configurations and noticed this only occurs when I have overlapping particles in my rigid body. I also looked through the fix rigid documentation but could not find anything mentioning this.
Any help understanding what is going on here is greatly appreciated.

Thank you for your time,